As a capacitor having a high capacitance and a low ESR used in various electronic equipments, an aluminum solid electrolytic capacitor and a tantalum solid electrolytic capacitor have been known.
A solid electrolytic capacitor has an aluminum foil having minute pores on a surface layer thereof or a sintered body of tantalum powder having minute pores inside as one electrode (conductor), and is produced by sealing a solid electrolytic capacitor element constituted by a dielectric layer formed on the surface layer of the electrode, another electrode (a semiconductor, in general) provided on the dielectric layer, and an electrode layer laminated on the another electrode.
An organic material and an inorganic material are used as the semiconductor layer, and a conductive polymer is preferably used in consideration of the heat resistance and the low ESR characteristics of the capacitor produced. The conductive polymer is such a polymer having a high conductivity of from 10−2 to 103 S·cm−1, and a high conductivity is exhibited by adding an electron donative compound called as a dopant to a polymer having a planar conjugated double bond (in general, an insulating material or a polymer exhibiting an extremely low conductivity). Specific examples of the method for forming a conductive polymer as the semiconductor layer include such a method that a monomer capable of becoming a conductive polymer is polymerized by feeding a suitable oxidizing agent or electrons in the presence of a dopant in the minute pores of the conductor. The dopant is incorporated upon polymerization of the monomer to obtain a conductive polymer.
U.S. Pat. No. 1,945,358 and U.S. Pat. No. 2,811,648 disclose a method of forming a semiconductor layer by forming a chemical polymerization layer by using an oxidizing agent and then laminating an electrolytic polymerization layer by an electrification method.